Product provided with a coding pattern and apparatus and method for reading the pattern

ABSTRACT

A product provided with a coding pattern which comprises a plurality of marks, each of which represents one of at least two different values, and which further comprises a plurality of nominal positions, each of said plurality of marks being associated with a nominal position and the value of each mark being determined by its location relative to its nominal position. The invention also comprises use of the product.

This application is a continuation of U.S. Non-Provisional applicationSer. No. 12/461,303, filed on Aug. 6, 2009, (now allowed), which is acontinuation of U.S. Non-Provisional application Ser. No. 11/268,562,filed Nov. 8, 2005, (now U.S. Pat. No. 7,588,191) and claims the benefitof U.S. Non-Provisional application Ser. No. 10/714,894, filed on Nov.18, 2003, (now U.S. Pat. No. 7,281,668), U.S. Non-Provisionalapplication Ser. No. 09/676,914, filed on Oct. 2, 2000 and issued asU.S. Pat. No. 6,663,008 on Dec. 16, 2003; U.S. Provisional ApplicationNo. 60/157,967, filed Oct. 6, 1999; and Application No. SE 9903541-2,filed in Sweden on Oct. 1, 1999, which are all incorporated herein byreference.

FIELD OF THE INVENTION

This invention concerns a product which is provided with a codingpattern, which comprises a number of marks, each of which represents oneof at least two different values. The invention also concerns use ofsuch a coding pattern.

BACKGROUND OF THE INVENTION

Storing coded information on a surface by means of different types ofmarks is already known.

U.S. Pat. No. 5,852,434 describes, for example, a position-codingpattern which codes X-Y-coordinates for a number of positions on awriting surface. The position-coding pattern makes it possible for auser to record electronically graphic information which is created on awriting surface by continuously reading the position-coding pattern.

Three examples of the construction of the position-coding pattern aregiven in U.S. Pat. No. 5,852,434. In the first example the patternconsists of symbols, each of which is constructed of three concentriccircles. The outer circle represents the X-coordinate and the middlecircle the Y-coordinate. Both the outer circles are additionally dividedinto 16 parts which, depending upon whether they are filled in or not,indicate different numbers. This means that each pair of coordinates X,Y is coded by a complex symbol with a particular appearance.

In the second example, the coordinates of each point on the writingsurface are given by means of barcodes, a bar-code for the X-coordinatebeing shown above a barcode for the Y-coordinate.

A checkered pattern which can be used to code the X- and Y-coordinatesis given as a third example. However, there is no explanation as to howthe checkered pattern is constructed or how it can be converted intocoordinates.

A problem with the known pattern is that it is constructed of complexsymbols and the smaller these symbols are made, the more difficult it isto produce the patterned writing surface and the greater the risk ofincorrect position determinations, while the larger the symbols aremade, the poorer the position resolution becomes.

A further problem is that the processing of the detected position-codingpattern becomes rather complicated, due to the fact that a processor hasto interpret complex symbols.

An additional problem is that the detector or sensor which is to recordthe position-coding pattern must be constructed in such a way that itcan record four symbols at the same time so that it is certain to coverat least one symbol in its entirety, which is necessary in order for theposition determination to be able to be carried out. The ratio betweenthe required sensor surface and the surface of the position-codingpattern which defines a position is thus large.

In EP 0 578 692 a position-coding pattern is described which isconstructed of cells in the form of squares. The value of the cells isdetermined by their appearance, for example their color. The cells canbe separated by separation zones so that two adjacent cells with thesame color can be distinguished. The position-coding pattern differsfrom that according to U.S. Pat. No. 5,852,434 in that a particularnumber of cells, that is symbols, together code a position. The codingis in addition floating, which means that an arbitrary partial surfaceof the pattern which contains the above-mentioned number of cells codesa position. Each cell thus contributes to the coding of severalpositions. In this way the ratio between the required sensor surface andthe part of the position-coding pattern which defines a position is lessthan in the above-mentioned US patent. In addition, each cell is lesscomplex and therefore the processor which is to decode theposition-coding pattern needs to be able to recognize fewer differentelements. However, the processor needs to be able to locate anddistinguish at least two different cells.

EP 0 171 284 B1 shows another floating position-coding pattern which isconstructed of horizontal and vertical lines, the vertical lines codingthe position in the horizontal direction and the horizontal lines codingthe position in the vertical direction. The lines can be found inpositions which are a multiple of 1 mm. The presence of a line in such aposition codes a one (1), the absence of a line in such a position codesa zero (0).

It is, however, difficult to record and decode a pattern of lines, asthe intersections between the lines can be difficult to record. Inaddition, it is often the case that the sensor is not held parallel tothe base, which results in a perspective where the lines are no longerparallel. It can then be difficult to determine whether a line isactually missing. In addition, there must not be too many missingconsecutive lines, as difficulties can then arise in the decoding.Furthermore, the information content is small.

Applicant's Swedish Patent Application SE 9901954-9, which was filed on28 May 1999 and which was not publicly available at the time of filingthe present application and thereby does not constitute prior art,describes an additional position-coding pattern of the floating type inwhich the position information is coded graphically by means of dots ofa first and a second size, a dot of the first size corresponding to azero (0) and a dot of the second size corresponding to a one (1). Aplurality of dots together code the coordinates for a position.

It is a general desire that coding patterns which are used to storeinformation on a surface must be able to code a lot of information perunit area and must be simple to detect and decode even when subjected tointerference of difference kinds.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a productwith a coding pattern which fulfils this requirement to at least asgreat an extent as the known coding patterns.

The invention concerns more specifically a product which is providedwith a coding pattern, which comprises a plurality of marks, each ofwhich represents one of at least two different values. The codingpattern comprises a plurality of nominal positions, each of said marksbeing associated with one of said plurality of nominal positions and thevalue of each mark being determined by its location relative to itsnominal position.

In prior art the coding is usually carried out by means of theappearance of one or more symbols or marks, the value of each symbol ormark being determined by its appearance. The device which decodes thecoding pattern must consequently be able to recognize different symbolsor marks, which increases the sensitivity to interference and makes thedecoding more difficult.

According to the present invention, the value of each mark is determinedinstead by how it is located relative to its nominal position. As thevalue is based on the location of the mark, all the marks can have anidentical appearance. The coding pattern is consequently simple to applyon the product. Furthermore, the detection of the marks is simple tocarry out and unaffected by the presence of other marks on the productwhich are not part of the coding pattern. In addition, the codingpattern can be realized more simply using other technology than opticaltechnology, for example as a chemical, electrical or mechanical pattern.The design of the mark also means that a product which is provided witha coding pattern will be more esthetically pleasing when the mark isoptically readable. Finally, it is possible to have a large distancebetween the marks in relation to the density of the information, whichmeans that the coding pattern is less sensitive to motion blur which canarise during the reading.

By nominal position is meant in this connection a position which isdetectable and relative to which the mark can be located in differentways. The nominal positions can be marked on the product, but they canalso be virtual and detectable indirectly.

It should also be pointed out that the value which a mark represents ispreferably a numerical value, but can also be a character value, such aletter or some kind of symbol.

The location of the mark can preferably be determined by its center ofgravity, which makes possible the use of marks of irregular shape andreduces the demands when applying the pattern on the product.

In a preferred embodiment, each nominal position is allocated a mark.The advantage is hereby obtained that all values are coded by a mark.The absence of a mark thus always constitutes an error.

The marks can be placed both in the nominal position and outside thesame. A possible representation of a binary pattern could, for example,be that a mark in the nominal position represents a zero and a markoutside the nominal position represents a one, or vice versa.

In a preferred embodiment, however, essentially all the marks aredisplaced relative to their nominal position. In this way the pattern israndom, while at the same time it is so uniform that it appears even tothe eye.

A few marks should, however, be able to be in their nominal position inorder to indicate some specific parameters, for example the position ofthe virtual raster.

In addition, in a preferred embodiment, essentially all the marks aredisplaced the same distance relative to their nominal position. If it isknown where the nominal position is located, it is sufficient to lookfor a mark at a certain distance from the nominal position, whichfacilitates the locating of the marks and reduces the risk of errors. Inaddition, it is sufficient to detect that there is a mark at therelevant distance from the nominal position. The appearance of this markis of subordinate significance, which reduces the need for precision inapplying the pattern on the product.

In a particularly preferred embodiment, each mark is displaced in one offour orthogonal directions relative to its nominal position. By knowingthe nominal position the mark accordingly only needs to be looked for infour different directions. This facilitates and speeds up the locatingof the marks. In addition, it reduces the risk of errors, as marks whichare not part of the pattern and which are situated in other positionsthan along the four orthogonal directions are not detected and therebydo not run the risk of affecting the decoding of the pattern.

In order for it to be possible to determine the locations of the marksrelative to the nominal positions, the nominal positions must be known.For this purpose the coding pattern preferably comprises a raster withraster lines, where the intersections of the raster lines define thenominal positions of the marks. The nominal positions are thus regularlyarranged on the product. This facilitates the detection and reduces therisk of error. In addition, it makes possible the use of a virtualraster.

In a preferred embodiment, the distance between the raster lines isapproximately 250 μm to 300 μm, preferably 300 μm. This makes possible ahigh density of information, but still with reliable detection.

In a preferred embodiment, the raster lines also form a rectangular,preferably square, grid. In the latter case, the distance between theraster lines is thus the same in both directions.

In a preferred embodiment, each mark is additionally displaced along oneof the raster lines. When the raster is known, the marks can thus belocated in an efficient way by searching along the well-defineddirections which the raster lines represent.

In a preferred embodiment, each mark is displaced from its nominalposition by a distance which is ¼ to ⅛, preferably ⅙, of the distancebetween the raster lines. If the displacement is approximately ⅙ of theraster line interval, it is relatively easy to determine to whichnominal position the mark belongs. If the displacement is less thanapproximately ⅛, it can be difficult to detect, that is the resolutionrequirement is too great. If the displacement is more than approximately¼, it can be difficult to determine to which nominal position the markbelongs. This applies in particular if the representation of the codingpattern recorded by the sensor or detector is distorted, which forexample can occur if an optical sensor is held at an angle relative tothe surface on which the coding pattern is arranged. With theabove-mentioned preferred raster line interval of 300 μm, the preferreddisplacement is thus 50 μm.

The raster with the raster lines can be indicated on the surface in sucha way that it can be read directly by the device which detects themarks. In this case, however, the raster must also be able to bedetected by the device and distinguished from the marks. In a preferredembodiment, the raster is instead virtual, which means that it is notmarked on the product in any way, but can be located from the locationsof the marks. Instead of being read from the product, it is thusdetermined indirectly by means of the marks.

As already mentioned, essentially all the marks in a preferredembodiment have an essentially identical appearance. This makes itsimpler to arrange them on the product.

The marks have preferably some simple geometric shape. They are thusadvantageously approximately circular, triangular or rectangular. Theycan be filled-in or not, but the former is preferable as detection isthen simpler.

The mark should not cover its nominal position and should therefore nothave a larger diameter than twice the displacement, that is 200%. Thisis, however, not critical, as a certain amount of overlapping ispermissible, for example 240%. The smallest size is determined in thefirst place by the resolution of the sensor and the requirements of theprinting process used to produce the pattern. However, in practice themarks should not have a smaller diameter than approximately 50% of thedisplacement, in order to avoid problems with particles and noise in thesensor.

The coding pattern can be realized with any parameters which can be usedto produce marks of the above-mentioned type which can be detected by adetector. The parameters can be electrical or chemical or of some othertype. The coding pattern is, however, preferably optically readable inorder for it to be simpler to arrange on the product. It can, forexample, be printed on the product.

In a preferred embodiment, the coding pattern is readable by infraredlight. In this way information which is not readable by infrared lightcan be overlaid on the coding pattern without interfering with thereading of this.

In a preferred embodiment, the marks constitute 0.25% to 20%, preferablyapproximately 9%, of the surface which is taken up by the codingpattern. If the pattern is printed, for example, on a sheet of whitepaper, it will in this case only result in a pale gray shading of thepaper, which means that it will appear as essentially normal paper.

The coding pattern is preferably a position-coding pattern which codes aplurality of positions on the product, each position being coded bymeans of a plurality of marks. The coding pattern can, however, also beused to code other information.

The product can be any product which can be provided with a codingpattern. It does not need to be a physical product, but can also beelectronic, for example an image or a surface on a computer screen onwhich the coding pattern is overlaid in electronic form.

According to another aspect of the invention, this concerns use of acoding pattern which comprises a plurality of marks, each of whichrepresents one of at least two different values, and a plurality ofnominal positions, each of said plurality of marks being associated withone of said plurality of nominal positions and the value of each markbeing determined by its location relative to its nominal position.

The advantages of the use of such a pattern are apparent from thediscussion of the coding pattern on the product. The features which arementioned for the coding pattern on the product also apply, of course,to the use of the coding pattern. The use can, for example, consist ofprinting out the coding pattern on a product, storing the coding patternin electronic form or decoding the coding pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail byway of an embodiment and with reference to the accompanying drawings, inwhich

FIG. 1 shows schematically an embodiment of a product which is providedwith a position-coding pattern;

FIGS. 2 a-2 d show schematically how the marks can be designed andpositioned in an embodiment of the invention;

FIG. 3 shows schematically an example of 4*4 symbols which are used tocode a position;

FIG. 4 shows schematically a device which can be used for positiondetermination;

FIG. 5 shows schematically a position-coding pattern with triangularraster;

FIG. 6 shows schematically a position-coding pattern with hexagonalraster; and

FIG. 7 including FIG. 7( a)-(d) illustrate alternative mark positionsfor alternative embodiments of the position coding patterns described inthe present application.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows a part of a product in the form of a sheet of paper 1,which on at least part of its surface 2 is provided with an opticallyreadable position-coding pattern 3 which makes possible positiondetermination.

The position-coding pattern comprises marks 4, which are systematicallyarranged across the surface 2, so that it has a “patterned” appearance.The sheet of paper has an X-coordinate axis and a Y-coordinate axis. Theposition determination can be carried out on the whole surface of theproduct. In other cases the surface which enables position determinationcan constitute a small part of the product.

The pattern can, for example, be used to provide an electronicrepresentation of information which is written or drawn on the surface.The electronic representation can be provided, while writing on thesurface with a pen, by continuously determining the position of the penon the sheet of paper by reading the position-coding pattern.

The position-coding pattern comprises a virtual raster, which is thusneither visible to the eye nor can be detected directly by a devicewhich is to determine positions on the surface, and a plurality of marks4, each of which, depending upon its location, represents one of fourvalues “1” to “4” as described below. In this connection it should bepointed out that for the sake of clarity the position-coding pattern inFIG. 1 is greatly enlarged. In addition, it is shown arranged only onpart of the sheet of paper.

The position-coding pattern is so arranged that the position of apartial surface on the total writing surface is determined unambiguouslyby the marks on this partial surface. A first and a second partialsurface 5 a, 5 b are shown by broken lines in FIG. 1. The second partialsurface partly overlaps the first partial surface. The part of theposition-coding pattern (here 4*4 marks) on the first partial surface 5a codes a first position and the part of the position-coding pattern onthe second partial surface 5 b codes a second position. Theposition-coding pattern is thus partly the same for the adjoining firstand second positions. Such a position-coding pattern is called“floating” in this application. Each partial surface codes a specificposition.

FIGS. 2 a-d show how a mark can be designed and how it can be locatedrelative to its nominal position 6. The nominal position 6, which alsocan be called a raster point, is represented by the intersection of theraster lines 8. The mark 7 has the shape of a circular dot. A mark 7 anda raster point 6 can together be said to constitute a symbol.

In one embodiment, the distance between the raster lines is 300 μm andthe angle between the raster lines is 90 degrees. Other raster intervalsare possible, for example 254 μm to suit printers and scanners whichoften have a resolution which is a multiple of 100 dpi, whichcorresponds to a distance between points of 25.4 mm/100, that is 254 μm.

The value of the mark thus depends upon where the mark is locatedrelative to the nominal position. In the example in FIG. 2 there arefour possible locations, one on each of the raster lines extending fromthe nominal position. The displacement from the nominal position is thesame size for all values.

Each mark 7 is displaced relative to its nominal position 6, that is nomark is located at the nominal position. In addition, there is only onemark per nominal position and this mark is displaced relative to itsnominal position. This applies to the marks which make up the pattern.There can be other marks on the surface which are not part of thepattern and thus do not contribute to the coding. Such marks can bespecks of dust, unintentional points or marks and intentional marks,from for example a picture or figure on the surface. Because theposition of the pattern marks on the surface is so well defined, thepattern is unaffected by such interference.

In one embodiment, the marks are displaced by 50 μm relative to thenominal positions 6 along the raster lines 8. The displacement ispreferably ⅙ of the raster interval; as it is then relatively easy todetermine to which nominal position a particular mark belongs. Thedisplacement should be at least approximately ⅛ of the raster interval,otherwise it becomes difficult to determine a displacement, that is therequirement for resolution becomes great. On the other hand, thedisplacement should be less than approximately ¼ of the raster intervalin order for it to be possible to determine to which nominal position amark belongs.

The displacement does not need to be along the raster line, but themarks can be positioned in separate quadrants. However, if the marks aredisplaced along the raster lines, the advantage is obtained that thedistance between the marks has a minimum which can be used to recreatethe raster lines, as described in greater detail below.

Each mark consists of a more or less circular dot with a radius which isapproximately the same size as the displacement or somewhat less. Theradius can be 25% to 120% of the displacement. If the radius is muchlarger than the displacement, it can be difficult to determine theraster lines. If the radius is too small, a greater resolution isrequired to record the marks.

The marks do not need to be circular or round, but any suitable shapecan be used, such as square or triangular, etc.

Normally, each mark covers a plurality of pixels on a sensor chip and,in one embodiment, the center of gravity of these pixels is recorded orcalculated and used in the subsequent processing. Therefore the preciseshape of the mark is of minor significance. Thus relatively simpleprinting processes can be used, provided it can be ensured that thecenter of gravity of the mark has the required displacement.

In the following, the mark in FIG. 2 a represents the value 1, in FIG. 2b the value 2, in FIG. 2 c the value 3 and in FIG. 2 d the value 4.

Each mark can thus represent one of four values “1 to 4”. This meansthat the position-coding pattern can be divided into a first positioncode for the x-coordinate and a second position code for they-coordinate. The division is carried out as follows:

Mark value x-code y-code 1 1 1 2 0 1 3 1 0 4 0 0

The value of each mark is thus converted into a first value, here bit,for the x-code and a second value, here bit, for the y-code. In this waytwo completely independent bit patterns are obtained by means of thepattern. Conversely, two or more bit patterns can be combined into acommon pattern which is coded graphically by means of a plurality ofmarks in accordance with FIG. 2.

Each position is coded by means of a plurality of marks. In thisexample, 4*4 marks are used to code a position in two dimensions, thatis an x-coordinate and a y-coordinate.

The position code is constructed by means of a number series of ones andzeros, a bit series, which has the characteristic that no four bit longbit sequence occurs more than once in the bit series. The bit series iscyclic, which means that the characteristic also applies when the end ofthe series is connected to its beginning. A four-bit sequence has thusalways an unambiguously determined position number in the bit series.

The bit series can be a maximum of 16 bits long if, it is to have thecharacteristic described above for bit sequences of four bits. In thisexample, however, only a seven bit long bit series is used, as follows:

-   -   “0 0 0 1 0 1 0”.

This bit series contains seven unique bit sequences of four bits whichcode a position number in the series as follows:

Position number in the series Sequence 0 0001 1 0010 2 0101 3 1010 40100 5 1000 6 0000

To code the x-coordinate, the bit series is written sequentially incolumns over all the surface which is to be coded, where the left columnK_(o) corresponds to the x-coordinate zero (0). In one column the bitseries can thus be repeated several times in succession.

The coding is based on differences or position displacements betweenadjacent bit series in adjacent columns. The size of the difference isdetermined by the position number (that is the bit sequence) in the bitseries with which the adjacent columns commence.

More precisely, if one takes the difference Δ_(n) modulo seven between,on the one hand, a position number which is coded by a four-bit sequencein a first column K_(n) and which can thus have the value 0 to 6, and,on the other hand, a position number which is coded by an adjacentfour-bit sequence at a corresponding “height” in an adjacent columnK_(n+1), the difference will be the same regardless of where, that is atwhat “height”, along the two columns the difference is created. Usingthe difference between the position numbers for two bit sequences in twoadjacent columns, it is thus possible to code an x-coordinate which isindependent of and constant for all y-coordinates.

As each position on the surface is coded by a partial surface consistingof 4*4 marks in this example, there are four vertical bit sequencesavailable and thus three differences, each with the value 0 to 6, forcoding the x-coordinate.

The pattern is divided into code windows F with the characteristic thateach code window consists of 4*4 marks. There are thus four horizontalbit sequences and four vertical bit sequences available, so that threedifferences can be created in the x-direction and four positions can beobtained in the y-direction. These three differences and four positionscode the position of the partial surface in the x-direction and they-direction. Adjacent windows in the x-direction have a common column,see FIG. 1. Thus the first code window F_(o,o) contains bit sequencesfrom the columns K₀, K₁, K₂, K₃, and bit sequences from the rows R_(o),R₁, R₂, R₃. As differences are used in the x-direction, the next windowdiagonally in the x-direction and y-direction, the window F_(1,1),contains bit sequences from the columns K₃, K₄, K₅, K₆, and the rows R₄,R₅, R₆, R₇. Considering the coding in just the x-direction, the codewindow can be considered to have an unlimited extent in the y-direction.Correspondingly, considering the coding in just the y-direction, thecode window can be considered to have an unlimited extent in thex-direction. Such a first and second code window with unlimited extentin the y-direction and x-direction respectively together form a codewindow of the type shown in FIG. 1, for example F_(o,o).

Each window has window coordinates F_(X), which give the position of thewindow in the x-direction, and F_(Y), which give the position of thewindow in the y-direction. Thus the correspondence between the windowsand columns is as follows.

K_(i)=3F_(X)

R_(j)=4F_(Y)

The coding is carried out in such a way that for the three differences,one of the differences Δ₀ always has the value 1 or 2, which indicatesthe least significant digit S_(o) for the number which represents theposition of the code window in the x-direction, and both the otherdifferences Δ₁, Δ₂, have values in the range 3 to 6, which indicates thetwo most significant digits S₁, S₂, for the coordinate of the codewindow. Thus no difference can be zero for the x-coordinates, as thatwould result in too symmetrical a code pattern. In other words, thecolumns are coded so that the differences are as follows; (3 to 6); (3to 6); (1 to 2); (3 to 6); (3 to 6); (1 to 2); (3 to 6); (3 to 6); (1 to2); (3 to 6); (3 to 6); . . .

Each x-coordinate is thus coded by two differences Δ₁, Δ₂ of between 3and 6 and a subsequent difference Δ₀ which is 1 or 2. By subtracting one(1) from the least difference Δ₀ and three (3) from the otherdifferences, three digits are obtained, S₂, S₁, S₀, which in a mixedbase directly give the position number of the code window in thex-direction, from which the x-coordinate can then be determineddirectly, as shown in the example below. The position number of the codewindow is:

S₂*(4*2)+S₁*2+S₀*1

Using the principle described above, it is thus possible to code codewindows 0, 1, 2, . . . , 31, using a position number for the code windowconsisting of three digits which are represented by three differences.These differences are coded by a bit pattern which is based on thenumber series above. The bit pattern can finally be coded graphically bymeans of the marks in FIG. 2.

In many cases, when a partial surface is recorded consisting of 4*4marks, a complete position number which codes the x-coordinate will notbe obtained, but parts of two position numbers will, as the partialsurface in many cases does not coincide with one code window but coversparts of two adjacent code windows in the x-direction. However, as thedifference for the least significant digit S₀ of each number is always 1or 2, a complete position number can easily be reconstructed, as it isknown what digit is the least significant.

The y-coordinates are coded in accordance with approximately the sameprinciple as that used for the x-coordinates by means of code windows.The cyclic number series, that is the same number series as is used forthe x.-coding, is written repeatedly in horizontal rows across thesurface which is to be position coded. Precisely as for thex-coordinates, the rows are made to start in different positions, thatis with different bit sequences, in the number series. For they-coordinates, however, differences are not used, but the coordinatesare coded by values which are based on the start position of the numberseries in each row. When the x-coordinate has been determined for apartial surface with 4*4 marks, the start positions in the number seriescan in fact be determined for the rows which are included in the y-codefor the 4*4 marks.

In the y-code, the least significant digit S₀ is determined by lettingthis be the only digit which has a value in a particular range. In thisexample, a row of four starts in position 0 to 1 in the number series,in order to indicate that this row concerns the least significant digitS₀ in a code window, and the three other rows start in any of thepositions 2 to 6 in order to indicate the other digits S₁ S₂ S₃ in thecode window. In the y-direction there is thus a series of values asfollows:

(2 to 6); (2 to 6); (2 to 6); (0 to 1); (2 to 6); (2 to 6); (2 to 6); (0to 1); (2 to 6); . . .

Each code window is thus coded by three values between 2 and 6 and asubsequent value between 0 and 1.

If zero (0) is subtracted from the low value and two (2) from the othervalues, a position in the y-direction S₃ S₂ S₁ S₀ in mixed base isobtained, in a way similarly to the x-direction, from which the positionnumber of the code window can be determined directly, which is:

S₃*(5*5*2)+S₂*(5*2)+S₁*2+S₀*1

Using the method above, it is possible to code 4*4*2=32 position numbersin the x-direction for the code windows. Each code window comprises bitsequences from three columns, which gives 3*32=96 columns orx-coordinates. In addition, it is possible to code 5*5*5*2=250 positionnumbers in the y-direction for the code windows. Each such positionnumber comprises horizontal bit sequences from 4 rows, which gives4*250=1000 rows or y-coordinates. In total it is thus possible to code96000 coordinate positions.

As the x-coding is based on differences, it is, however, possible toselect the position in which the first number series in the first codewindow starts. If it is taken into account that this first number seriescan start in seven different positions, it is possible to code7*96000=672000 positions. The start position of the first number seriesin the first column K₀ can be calculated when the x- and y-coordinateshave been determined. The above-mentioned seven different startpositions for the first series can code different pages or writingsurfaces of a product.

Theoretically, a partial surface with 4*4 symbols, which each have fourvalues, can code 4^(4*4) positions, that is 4,294,967,296 positions. Inorder to make possible floating determination of the position of apartial surface, there is thus a redundancy factor in excess of 6000(4294967296/672000).

The redundancy consists partly in the restrictions on the size of thedifferences, and partly in only 7 bits out of 16 being used in theposition code. This latter fact can, however, be used to determine therotational position of the partial surface. If the next bit in the bitseries is added to the four-bit sequence, a five-bit sequence isobtained. The fifth bit is obtained by reading the adjacent bitimmediately outside the partial surface which is being used. Such anadditional bit is often easily available.

The partial surface which is read by the sensor can have four differentrotational positions, rotated through 0, 90, 180 or 270 degrees relativeto the code window. In those cases where the partial surface is rotated,the reading of the code will, however, be such that the code read willbe inverted and reversed in either the x-direction or the y-direction orboth, in comparison to the case where if it had been read at 0 degrees.This assumes, however, that a slightly different decoding of the valueof the marks is used according to the table below.

Mark value x-code y-code 1 0 0 2 1 0 3 1 1 4 0 1

The above-mentioned five-bit sequence has the characteristic that itonly occurs the right way round and not in inverted and reversed form inthe seven-bit series. This is apparent from the fact that the bit series(0 0 0 1 0 1 0) contains only two “ones”. Therefore all five-bitsequences must contain at least three zeros, which after inversion (andreversing, if any) results in three ones, which cannot occur. Thus if afive-bit sequence is found which does not have a position number in thebit series, it can be concluded that the partial surface should probablybe rotated and the new position tested.

In order to provide further illustrations of the invention according tothis embodiment, here follows a specific example which is based on thedescribed embodiment of the position code.

FIG. 3 shows an example of an image with 4*4 marks which are read by adevice for position determination.

These 4*4 marks have the following values:

-   -   4 4 4 2    -   3 2 3 4    -   4 4 2 4    -   1 3 2 4        These values represent the following binary x- and y-codes:

x-code: v-code: 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 0 0 0 0 0 1 0 1 1 0 01 0 1 0

The vertical bit sequences in the x-code code the following positions inthe bit series: 2 0 4 6. The differences between the columns are −2 4 2,which modulo 7 gives: 5 4 2, which in mixed base codes the positionnumber of the code window: (5-3)*8+(4-3)*2+(2-1)=16+2+1=19. The firstcoded code window has the position number 0. Thus the difference whichlies in the range 1 to 2 and which appears in the 4*4 marks of thepartial surface is the twentieth such difference. As additionally thereare in total three columns for each such difference and there is astarting column, the vertical sequence furthest to the right in the 4*4x-code belongs to the 61st column (column 60) in the x-code (3*20+1=61)and the vertical sequence furthest to the left belongs to the 58thcolumn (column 57).

The horizontal bit sequences in the y-code code the positions 0 4 1 3 inthe number series. As these horizontal bit sequences start in the 58thcolumn, the start position of the rows is the value of these minus 57modulo 7, which gives the start positions 6 3 0 2. Converted to digitsin the mixed base, this becomes 6-2, 3-2, 0-0, 2-2=4 1 0 0, where thethird digit is the least significant digit in the number concerned. Thefourth digit is then the most significant digit in the next number. Itmust in this case be the same as in the number concerned. (The exceptionis when the number concerned consists of highest possible digits in allpositions. Then it is known that the beginning of the next number is onegreater than the beginning of the number concerned.)

The position number is in mixed base 0*50+4*10+1*2+0*1=42.

The third horizontal bit sequence in the y-code thus belongs to the 43rdcode window which has a start position 0 or 1, and as there are fourrows in total for each such code window, the third row is number43*4=172.

In this example, the position of the top left corner of the partialsurface with 4*4 marks is (58,170).

As the vertical bit sequences in the x-code in the 4*4 group start atrow 170, the whole pattern's x-columns start in the positions of thenumber series ((2 0 4 6) −169) modulo 7=1 6 3 5. Between the last startposition (5) and the first start position the numbers 0-19 are coded inthe mixed base, and by adding the representations of the numbers 0-19 inthe mixed base the total difference between these columns is obtained. Anaive algorithm for doing this is to generate these twenty numbers anddirectly add their digits. Call the sum obtained s. The page or writingsurface is then given by (5-s) modulo7.

An alternative method for determining which bit is the least significantin a partial surface in order to be able to identify a code window inthis way is as follows. The least significant bit (LSB) is defined asthe digit which is the lowest in a partial surface's differences or rowposition number. In this way the reduction (redundancy) of the maximumuseable number of coordinates is relatively small. For example, thefirst code windows in the x-direction in the example above can all haveLSB=1 and other digits between 2 and 6, which gives 25 code windows, thenext can have LSB=2 and other digits between 3 and 6, which gives 16code windows, the next can have LSB=3 and other digits between 4 and 6,which gives 9 code windows, the next can have LSB=4 and other digitsbetween 5 and 6, which gives 4 code windows, the next can have LSB=5 andother digits 6, which gives 1 code window, that is a total of 55 codewindows, compared to 32 in the example above.

In the example above, an embodiment has been described where each codewindow is coded by 4*4 marks and a number series with 7 bits is used.This is of course only one example. Positions can be coded by more orfewer marks. There does not need to be the same number in bothdirections. The number series can be of different length and does notneed to be binary, but can be based on a different base, for example hexcode. Different number series can be used for coding in the x-directionand coding in the y-direction. The marks can represent different numbersof values.

In a practical example, a partial surface is used consisting of 6*6marks and where the bit series as a maximum can consist of 2⁶ bits, thatis 64 bits. However, a bit series consisting of 51 bits is used, andconsequently 51 positions, in order to have the ability to determine therotational position of the partial surface. An example of such a bitseries is:

0 0 0 0 0 1 1 0 0 0 1 1 1 1 10 1 0 1 0 1 1 0 1 1 0 0 1 1

0 1 0 0 0 1 0 1 0 0 1 1 1 0 1 1 1 1 0 0 1 0

Such a partial surface consisting of six by six marks can theoreticallycode 4^(6*6) positions, which with the above-mentioned raster dimensionof 0.3 mm is an extremely large surface.

In a similar way as described above for the seven-bit series, accordingto the present invention the characteristic is utilized that the partialsurface is enlarged to include one bit on each side of the partialsurface, at least at its center, so that for the third and fourth rowsin the partial surface of 6*6 symbols, 8 symbols are read, one on eachside of the partial surface, and similarly in the y-direction. Theabove-mentioned bit series which contains 51 bits has the characteristicthat a bit sequence of 6 bits occurs only once and that a bit sequenceof 8 bits which contains the above-mentioned bit sequence of 6 bitsoccurs only once and never in an inverted position or reversed andinverted. In this way, the rotational position of the partial surfacecan be determined by reading 8 bits in row 3, row 4, column 3 and/orcolumn 4. When the rotational position is known, the partial surface canbe rotated to the correct position before processing is continued.

It is desirable to obtain a pattern which is as random as possible, thatis where areas with excessive symmetry do not occur. It is desirable toobtain a pattern where a partial surface with 6*6 marks contains markswith all the different positions in accordance with FIGS. 2 a to 2 d. Inorder to increase the randomness further or avoid repetitivecharacteristics, a method can be used which is called “shuffle”. Eachhorizontal bit sequence starts in a predetermined start position.However, it is possible to displace the start position in the horizontaldirection for each row, if the displacement is known. This can becarried out by each least significant bit (LSB) being allocated aseparate displacement vector for the adjacent rows. The displacementvector states by how much each row is displaced in the horizontaldirection. Visually it can be regarded as if the y-axis in FIG. 1 is“spiky”.

In the example above, with a 4*4 code window, the displacement vectorcan be 1, 2, 4, 0 for LSB=0 and 2, 2, 3, 0 for LSB=1. This means thatafter subtracting the number 2 and 0 respectively, the abovedisplacement is to be subtracted (modulo five) from the bit sequence'sposition number, before the processing continues. In the example above,for the y-coordinate the digits 4 1 0 0 (S₂, S₁, S₀, S₄) are obtained inthe mixed base, where the second digit from the right is the leastsignificant digit, LSB. As the displacement vector 1, 2, 4, 0 is to beused (LSB=0) for the digits 4 and 1, 2 is subtracted from 4 to give S₂=2and 4 is subtracted from 1 (modulo five) to give S₁=2. The digit S₀=0remains unchanged (the displacement vector's component for the leastsignificant digit is always zero). Finally, the digit S₄ belongs to thenext code window, which must have LSB=1, that is the second displacementvector is to be used. Thus 2 is subtracted from 0 (modulo five) whichgives S₄=3.

A similar method can be used to change the codes for the x-coordinates.However, there is less need to change the x-coordinates, as they arealready relatively randomly distributed, as the difference zero is notused, in the example above.

In the example above, the mark is a dot. Naturally it can have adifferent appearance. It can, for example, consist of a line or anellipse, which starts at the virtual raster point and extends from thisto a particular position. Other symbols than a dot can be used, such asa square, rectangle, triangle, circle or ellipse, filled-in or not.

In the example above, the marks are used within a square partial surfacefor coding a position. The partial surface can be another shape, forexample hexagonal. The marks do not need to be arranged along the rasterlines in an orthogonal raster but can also be arranged in other manners,such as along the raster lines in a raster with 60 degree angles, etc. Apolar coordinate system can also be used.

Rasters in the form of triangles or hexagons can also be used, as shownin FIGS. 5 and 6. For example, a raster with triangles, see FIG. 5,enables each mark to be displaced in six different directions, whichprovides even greater possibilities, corresponding to 6_(6*6) partialsurface positions. For a hexagonal raster, FIG. 6, a honeycomb pattern,each mark can be displaced in three different directions along theraster lines.

As mentioned above, the marks do not need to be displaced along theraster lines but can be displaced in other directions, for example inorder to be located each in a separate quadrant when using a squareraster pattern. In the hexagonal raster pattern the marks can bedisplaced in four or more different directions, for example in sixdifferent directions along the raster lines and along lines which are at60 degrees to the raster lines.

In order for the position code to be able to be detected, it isnecessary for the virtual raster to be determined. This can be carriedout, in a square raster pattern, by examining the distance betweendifferent marks. The shortest distance between two marks must originatefrom two adjacent marks with the values 1 and 3 in the horizontaldirection or 2 and 4 in the vertical direction, so that the marks lie onthe same raster line between two raster points. When such a pair ofmarks has been detected, the associated raster points (the nominalpositions) can be determined using knowledge of the distance between theraster points and the displacement of the marks from the raster points.Once two raster points have been located, additional raster points canbe determined using the measured distance to other marks and fromknowledge of the distance between the raster points.

If the marks are displaced 50 μm along the raster lines, which are adistance of 300 μm apart, the least distance between two marks will be200 μm, for example between marks with the values 1 and 3. The nextsmallest distance arises between, for example, marks with the values 1and 2, and is 255 μm. There is therefore a relatively distinctdifference between the least and the next smallest distance. Thedifference to any diagonals is also great. However, if the displacementis larger than 50 μm, for example more than 75 μm (¼), diagonals cancause problems and it can be difficult to determine to which nominalposition a mark belongs. If the displacement is less than 50 μm, forexample less than approximately 35 μm (⅛), the least distance will be230 μm, which does not give a very large difference to the nextdistance, which is then 267 μm. In addition, the demands on the opticalreading increase.

The marks should not cover their own raster point and should thereforenot have a larger diameter than twice the displacement, that is 200%.This is, however, not critical, and a certain overlapping can bepermitted, for example 240%. The least size is determined in the firstplace by the resolution of the sensor and the demands of the printingprocess used to produce the pattern. However, the marks should not havea smaller diameter than approximately 50% of the displacement inpractice, in order to avoid problems with particles and noise in thesensor.

FIGS. 7( a) to 7(c) illustrate alternative mark positions described inthe present application. FIG. 7( a) illustrates the disclosed embodimentwhere a mark 7 typically used for information encoding is displaced in adirection along a raster line 8 from a nominal position 6. FIG. 7( b)illustrates an embodiment described above, where in addition to a mark 7used for information encoding, the pattern includes, at least somenominal positions, a nominal position identifying mark 6 a, in thisexample provided at the nominal point 6. FIG. 7( c) illustrates otherembodiments, for example the embodiment described above where the mark 7normally used for information encoding is positioned at the nominalposition 6 in order to, for example, represent a specific parameter or,in an embodiment described above, where a logical zero as compared to alogical one is positioned as mark 7 at the nominal position 6 asillustrated for example in FIG. 7( a). FIG. 7( d) illustrates theembodiment described in the preceding paragraph.

An embodiment of a device for position determination is shownschematically in FIG. 4. It comprises a casing 11 which hasapproximately the same shape as a pen. In the short side of the casingthere is an opening 12. The short side is intended to abut against or tobe held a short distance from the surface on which the positiondetermination is to be carried out.

The casing contains essentially an optics part, an electronic circuitrypart and a power supply.

The optics part comprises at least one light-emitting diode 13 forilluminating the surface which is to be imaged and a light-sensitivearea sensor 14, for example a CCD or CMOS sensor, for recording atwo-dimensional image. If required, the device can also contain anoptical system, such as a mirror and/or lens system. The light-emittingdiode can be an infrared light-emitting diode and the sensor can besensitive to infrared light.

The power supply for the device is obtained from a battery 15, which ismounted in a separate compartment in the casing.

The electronic circuitry part contains image-processing means 16 fordetermining a position on the basis of the image recorded by the sensor14 and in particular a processor unit with a processor which isprogrammed to read images from the sensor and carry out positiondetermination on the basis of these images.

In this embodiment, the device also comprises a pen point 17, with theaid of which ordinary pigment-based writing can be written on thesurface on which the position determination is to be carried out. Thepen point 17 is extendable and retractable so that the user can controlwhether or not it is to be used. In certain applications the device doesnot need to have a pen point at all.

The pigment-based writing is suitably of a type that is transparent toinfrared light and the marks suitably absorb infrared light. By using alight-emitting diode which emits infrared light and a sensor which issensitive to infrared light, the detection of the pattern can be carriedout without the above-mentioned writing interfering with the pattern.

The device also comprises buttons 18, by means of which the device canbe activated and controlled. It has also a transceiver 19 for wirelesstransmission, for example using infrared light, radio waves orultrasound, of information to and from the device. The device can alsocomprise a display 20 for displaying positions or recorded information.

A device for recording text is described in Applicant's Swedish PatentNo. 9604008-4. This device can be used for position determination if itis programmed in a suitable way. If it is to be used for pigment-basedwriting, then it must also be given a pen point.

The device can be divided between different physical casings, a firstcasing containing components which are required for recording images ofthe position-coding pattern and for transmitting these to componentswhich are contained in a second casing and which carry out the positiondetermination on the basis of the recorded image (s).

As mentioned, the position determination is carried out by a processorwhich thus must have software for locating marks in an image anddecoding them and for determining positions from the codes thusobtained. A person skilled in the art will be able, based on the exampleabove, to design software which carries out position determination onthe basis of an image of a part of a position-coding pattern.

In addition, on the basis of the description above, a person skilled inthe art will be able to design software for printing out theposition-coding pattern.

In the embodiment above, the pattern is optically readable and thesensor is therefore optical. As mentioned, the pattern can be based on aparameter other than an optical parameter. In such a case the sensormust of course be of a type which can read the parameter concerned.Examples of such parameters are chemical, acoustic or electromagneticmarks. Capacitive or inductive marks can also be used.

In the embodiment above, the raster is an orthogonal grid. It can alsohave other forms, such as a rhombic grid, for example with 60 degreeangles, a triangular or hexagonal grid, etc.

Displacement in more or less than four directions can be used; forexample displacement in three directions along a hexagonal virtualraster. In an orthogonal raster only two displacements can be used, inorder to facilitate the recreation of the raster. However, adisplacement in four directions is preferred, but six or eightdirections are also possible within the scope of the invention.

In the embodiment above, the longest possible cyclic number series isnot used. As a result, a degree of redundancy is obtained which can beused in various ways, for example to carry out error correcting, replacemissing or hidden marks, etc.

1-41. (canceled)
 42. A product for decoding information, the productcomprising: a sensor for detecting a plurality of marks on a surface toobtain associated mark location data, at least a first group of theplurality of marks being arranged on the surface so as to define aninvisible raster with predetermined dimensions, and wherein marks in atleast a second group of the plurality of marks are displaced from rasterlocations in a manner encoding information; and a processor forreceiving from the sensor at least some of the mark location data, theprocessor being configured to determine, using the received marklocation data, a position of the invisible raster on the surface, andthe processor being further configured to decode information based onpositions of at least some of the marks in the second group of theplurality of marks relative to locations on the invisible raster.